The present invention relates to a field-emission-type ion source such as a liquid-metal ion source, and more particularly to the protection of the heater and emitter tip of such ion source against material to be ionized.
The field-emission-type ion source indicated in U.S. Pat. No. 4,088,919 shows high brightness and can obtain a point source ion beam. Thus, it is anticipated that such ion source will be applied to the microanalysis and ion micro-beam lithography fields.
FIG. 1A and FIG. 1B show the schematic diagrams of conventional field-emission-type ion sources. In FIG. 1A a Joule heating ion emitter made by welding a needle-shaped emitter tip 2 to the top end of a hairpin-shaped filament 1 is shown. In the ion emitter diagram FIG. 1B, a through-hole 11 is prepared in the center of a boat-shaped heater 1 and the emitter tip 2 is inserted in this hole 11 to be Joule heated.
Both ion emitters store their material to be ionized 3 at the intersection (reservoir) of the heater 1 and the emitter tip 2. By Joule heating, the heater 1 in the above situation melts the material to be ionized 3, and through the balance between gravity and surface tension, the melted material to be ionized 3 flows down towards the end of the emitter tip 2, wetting the end of the emitter tip 2. A high electric field is formed at the end of the emitter tip 2 due to the extracting voltage supplied between the emitter tip 2 and the extracting electrode (not shown in the figure). As a result, ions of the material to be ionized are extracted from the end of the emitter tip 2.
However, in these conventional field-emission-type ion sources, the heater 1 and the emitter tip 2 are consumed rapidly because both the heater 1 and the emitter tip 2 react with the material to be ionized 3 during the source operation. The consumption of the heater 1 which is sustained at high temperatures during the operation is especially fast in comparison to that of the emitter tip 2. This often caused the disconnection of the heater 1 in a very short time. As a result, only short-life ion sources were realized. Some sort of measure is anticipated to lengthen their lives.